The invention relates to a modulation detection method comprising taking a sample from a received signal, determining reference constellation points, calculating path-specific error metrics from a sample point to the reference constellation points, and applying the path-specific error metrics corresponding to the reference constellation points to a modulation detector.
When information is transferred on a radio channel, the signal to be transmitted has to be subjected to modulation. Modulation converts the signal into a form in which it can be transmitted at radio frequency. A modulation method can be considered efficient for instance if it allows as much information as possible to be transferred at as narrow a frequency band as possible. Depending on the purpose of use, other features can also be emphasized. Modulation should also cause as little interference as possible to adjacent channels.
Modulation methods include e.g. xcfx80/4-DQPSK (xcfx80/4-shifted, Differential Quaternary Phase Shift Keying) modulation. This modulation method comprises eight phase states, but only four phase shifts. Allowed phase shifts (symbols) are xc2x1xcfx804 and xc2x13xcfx80/4. FIG. 3A shows the modulation phase shift diagram (constellation). Each phase shift corresponds to two bits to be transmitted. In other words, a digital signal modulates the carrier in two-bit periods so that a given phase shift during each symbol period corresponds to a given two-bit combination. A symbol period refers to a signal period employed in the transmission of two bits. Phase shifts corresponding to bit combinations 00, 01, 10 and 11 are xcfx80/4, 3xcfx80/4,xe2x88x92xcfx80/4 and xe2x88x923xcfx80/4. The symbol frequency used in e.g. the TETRA system (Terrestrial Trunked Radio) is 18 kHz, the bit frequency being 36 kHz.
When a signal is being received, it has to be demodulated in order for the information therein to be detected. However, a signal transferred over the radio path can be distorted in various ways, thus complicating modulation detection. Signal-impairing phenomena include e.g. noise and inter-symbol interference (ISI). A known solution to implement noise-tolerant modulation detection is an MLSE detector (Maximum Likelihood Sequence Estimation), which utilizes the generally known Viterbi algorithm, i.e. sequence detection. The Viterbi algorithm is a method of determining the most probable signal paths on the basis of path error metrics. Herein a signal path refers to different combinations of successive modulation symbols. In the MLSE detector such alternative signal paths are created and bits corresponding to a given signal period are detected at a delay based on the error metrics of a signal path composed of a plurality of successive modulation symbols, the path having the best metrics being selected from the alternative signal paths. The interference cancellation provided by the method is the better the more successive symbol periods the paths comprise. In fact, in this connection the term employed is detection depth, which refers to path length.
The problem in the above solution is that when the detection depth, i.e. the number of symbol periods included in the paths, is increased, the number of paths also increases. When e.g. xcfx80/4-DQPSK modulation is used, each symbol period comprises four different symbol alternatives, making the number of paths four-fold within each period. This, in turn, significantly increases the required computational capacity.
It is an object of the invention to provide a method which solves the above problems. The objects of the invention are achieved by a method which is characterized by selecting out of the reference constellation points the constellation points, preferably two of them, having the smallest error metrics, and applying only the error metrics corresponding thereto to the modulation detector.
The invention is based on selecting, during each symbol period, the constellation points corresponding to symbols closest to a sample point taken from a signal. In e.g. xcfx80/4-DQPSK modulation, the sample point can be assumed at a very high probability to be either of the two closest constellation points, and consequently these two points are selected for further processing. The two remaining constellation points are discarded from further processing.
An advantage of the method of the invention is that since improbable alternatives are discarded at an initial stage, the calculation required can be reduced. This, in turn, allows an increase in detection depth at a given computational capacity.
The invention also relates to an apparatus for modulation detection, the apparatus comprising: means for taking a sample from a signal, means for determining reference constellation points and calculating path-specific error metrics from a sample point to the reference constellation points, the apparatus being adapted to use the path-specific error metrics corresponding to the reference constellation points in the modulation detector for detecting bits, the apparatus being characterized by being adapted to select out of the reference constellation points the constellation points, preferably two of them, having the smallest error metrics and to use only the error metrics corresponding thereto in the modulation detector. Such an apparatus serves to achieve the advantages offered by the modulation detection method of the invention by a simple structure.